I. Field
This disclosure relates to wireless communication. More particularly, this disclosure relates to methods and systems for determining reverse-link data rates and scheduling in a multi-user wireless network.
II. Background
An orthogonal frequency division multiplex (OFDM) communication system effectively partitions an overall system bandwidth into multiple (NF) sub-carriers, which may also be referred to as frequency sub-channels, tones, or frequency bins. For an OFDM system, the data to be transmitted (i.e., the information bits) may be first encoded with a particular coding scheme to generate coded bits, and the coded bits can be further grouped into multi-bit symbols that are then mapped to modulation symbols. Each modulation symbol corresponds to a point in a signal constellation defined by a particular modulation scheme (e.g., M-PSK or M-QAM) used for data transmission. At each time interval that may be dependent on the bandwidth of each frequency sub-carrier, a modulation symbol may be transmitted on each of the NF frequency sub-carrier. OFDM may be used to combat inter-symbol interference (ISI) caused by frequency selective fading, which is characterized by different amounts of attenuation across the system bandwidth.
A multiple-input multiple-output (MIMO) communication system can employ multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT,NR}. Each of the NS independent channels may be referred to as a “spatial subchannel” of the MIMO channel and corresponds to a dimension. The MIMO system may likely provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
For a MIMO system that employs OFDM (i.e., a MIMO-OFDM system), NF frequency sub-carrier are available on each of the NS spatial sub-channels for data transmission. Each frequency sub-carrier of each spatial sub-channel may be referred to as a transmission channel. Thus, NF·NS transmission channels are available for data transmission between the NT transmit antennas and NR receive antennas.
For a MIMO-OFDM systems the NF frequency sub-channels of each spatial sub-channel may experience different channel conditions (e.g., different fading and multi-path effects), and may achieve different signal-to-noise-and-interference ratios (SNRs). Each transmitted modulated symbol may be affected by the response of the transmission channel on which the symbol was transmitted. Depending on the multi-path profile of the communication channel between the transmitter and receiver, the frequency response may vary wisely throughout the system bandwidth for each spatial sub-channel, and may further vary widely among the spatial sub-channels.
For a multi-path channel with a frequency response that is not flat, the information rate (i.e., the number of information bits per modulation symbol) that may be reliably transmitted on each transmission channel may be different from transmission channel to transmission channel. If the modulation symbols for a particular data packet are transmitted over multiple transmission channels, and if the response of these transmission channels varies widely, then these modulation symbols may be received with a wide range of SNRs. The SNR would then vary correspondingly across the entire received packet, which may then make it difficult to determine the proper rate for the data packet.
Since different transmitters and receivers may experience different (and possibly widely varying) channel conditions, it may be impractical to transmit data at the same transmit power and/or data rate to all receivers. Fixing these transmission parameters would likely result in a waste of transmit power, the use of sub-optimal data rates, and/or unreliable communication for various receivers, all of which leads to an undesirable decrease in system capacity.
The above discussion describes various problems with rate determination for reverse-link scheduling of any users. Unfortunately, in the case of MIMO communication systems, further difficulties arise with the simultaneous transmission of multiple streams from multiple antennas of multiple users.
Still further, the channel conditions may vary over time due to a wide variety of reasons. As a results the supported data rates for the transmission channels may also vary over time. Thus, the different transmission capabilities of the communication channels for different receivers, plus the multi-path and time-variant nature of these communication channels, and larger degrees of freedom in assigning different power and rates from different transmit antennas make it challenging to efficiently transmit data in a MIMO system.